The present invention relates, in general, to a control circuit, and, in particular, to a time delay circuit.
Several different designs exist for timer circuits. An example of a fairly simple design can be found in U.S. Pat. No. 4,409,640 entitled xe2x80x9cABORT CYCLE TIMER.xe2x80x9d U.S. Pat. No. 4,409,640 is hereby incorporated by reference into the specification of the present invention. This invention disclosed typical configurations of some Resistor-Capacitor (R-C) timer circuits. For these to operate, the circuits require a voltage source, at least one resistor, at least one capacitor, and a threshold circuit. The threshold circuit determines when the capacitor has charged to the appropriate voltage, and sends a signal. Since this occurs at regular intervals, a timer is created. The required threshold circuit is made up of several additional components. This makes the R-C timer circuit complicated, and thus more susceptible to failure.
A similar timer circuit is disclosed in U.S. Pat. No. 4,560,892 entitled xe2x80x9cONE-SHOT DELAY TIMER.xe2x80x9d U.S. Pat. No. 4,560,892 is hereby incorporated by reference into the specification of the present invention. In U.S. Pat. No. 4,560,892, an R-C timer circuit of the type described above is disclosed. The threshold circuit adds a number of resistors and an operational amplifier to the device. This makes the device complicated and more susceptible to failure. Further, the operational amplifier increases the cost a significant amount since it is also composed of transistors, resistors and capacitors.
The other major type of timer circuit is a digital timer circuit. An example of this type of circuit was disclosed in U.S. Pat. No. 5,303,279 entitled xe2x80x9cTIMER CIRCUIT.xe2x80x9d U.S. Pat. No. 5,303,279 is hereby incorporated by reference into the specification of the present invention. U.S. Pat. No. 5,303,279 uses a clock circuit, a bit counter, an overflow detection circuit, a delay circuit and a switch circuit to construct a timer circuit. The clock circuit sends bits to the bit counter. The bit counter, after receiving a certain number of bits, submits received bits to the overflow detector. The overflow detector then sends a signal to the delay circuit. The delay circuit holds the bits for a certain period of time before releasing them. This will occur at regular intervals, creating a timer. Each of these components is made up of complicated circuitry including resistors, transistors, and flip-flops, among other things. This decreases the reliability of the circuit and increases the overall cost. While the use of these types of circuits may be necessary for situations requiring high precision, the problems associated with them make them undesirable for less precise applications.
It is an object of the present invention to create a timer circuit utilizing the thermal runaway effect.
It is a further object of this invention to create a timer circuit utilizing the thermal diffusion effect capable of being used in an integrated circuit.
The present invention is a timer circuit utilizing the thermal runaway effect. The preferred timer circuit includes a switch, a resistor, a transistor and a protective device. A current is caused to flow in a first terminal of the transistor, which, in time, causes a current to flow in a second terminal of the transistor. This current flowing in the second terminal heats the transistor and increases its gain and leakage current. The increased gain causes a higher current to flow in the second terminal which heats the transistor further. This cycle eventually activates the protective device, which both acts as the time indicator and disconnects the power to the transistor to protect the transistor from destruction.
An alternate embodiment includes a switch, a resistor, a transistor, a heat source, two heat sensors and a comparator. The heat source rises in temperature due to the current flowing in the second terminal of the transistor. The two heat sensors rise in temperature due to the rise in temperature of the heat source, but one is located closer to the heat source than the other. A comparator compares the difference in temperature of the heat sensors, and when this difference reaches a user-definable value the comparator disconnects power to the transistor and activates a time indicator.